The present invention relates generally to image processing, and in particular, to an image processing device and method for reducing aliasing distortion due to undersampling of an image. The invention also relates to a technique for generating a high resolution image by increasing the number of pixels of a low resolution image.
With modern digital imaging techniques, analog video signals are sampled and stored as digital data for subsequent reproduction on a cathode ray tube (CRT) or other display. It is a design goal to display such images with as high a resolution and as little distortion as possible.
Images displayed on a CRT, such as images derived from television signals, video tape recorders (VTRs) or from digital versatile disc (DVD) players, are generated by repetitive scanning of electron beams in the horizontal direction. As illustrated in FIG. 25., each horizontal scan is performed from left to right, and after each scan, the beams are swept back to the extreme left and re-positioned vertically to commence the next horizontal scan. The fluorescent screen of the CRT is irradiated by three electron beams to illuminate respective blue, green and red phosphors distributed in small units on the screen. The phosphors produce points of light corresponding to the intensity of the electron beams, and the congregation of all the points produces an image. Hence, a displayed image can be considered a congregation of such points, that is, pixels.
Since a displayed image on a CRT derived from an analog signal is composed of a congregation of light emitting pixels, the image can be envisioned as a digital signal obtained by sampling the original image at the pixel positions. Thus, if the original analog image were sampled at a sufficient sampling interval in both horizontal and vertical directions to generate the same number of points as the number of pixels on a CRT, a collection of image data could be stored digitally. When subsequently reproduced, an image of nearly the same resolution is obtained as in a strictly analog recording/reproduction approach.
The sampling theorem dictates that an analog signal can be reconstructed completely from a set of uniformly spaced discrete samples thereof in time, provided that the signal is sampled at a rate of at least twice the highest frequency component of the signal. When sampling an original image, if the sampling theorem is not satisfied, aliasing distortion is generated in the displayed image. To correct for aliasing distortion, prefilters have been used to compensate for undersampling in the horizontal (scanning line) direction; however, such prefilters are not typically provided in the vertical direction. As such, aliasing distortion in the vertical direction is a common problem.
FIG. 26 illustrates aliasing distortion of a displayed image in the vertical direction. Four pixels P.sub.1 -P.sub.4 of a given column are shown in each of frames N and N+1. Signals S.sub.N and S.sub.N+1 represent the image level variation of the original image in the vertical direction for the given column, where the amplitude level is illustrated horizontally in the figure. Thus, for example, in frame N, the luminance of the image is higher for pixel P.sub.2 than for pixel P.sub.1. Now, if the highest spatial frequency component of the original image in the vertical direction has a period of less than twice the horizontal spacing between pixels, then the sampling theorem is not satisfied and aliasing distortion results. Such is the case for both signals S.sub.N and S.sub.N+1 in FIG. 26. For instance, signal S.sub.N ', which is an approximation for the sampled signal of frame N, is markedly different from the original signal S.sub.N. With the aliasing distortion, the high frequency component of the original signal will be lost during reproduction, even if a filter to remove aliasing is employed in the vertical direction. Such aliasing distortion may be a cause of degradation in signal processing such as Y/C separation, noise removal, quality improvement, and so forth.
It is noted that while undersampling of the image as just discussed will always result in diminished resolution, the effect on the viewer in terms of picture quality depends on how the scene changes from frame to frame. If a frame with aliasing distortion changes significantly from frame to frame, as is the case in FIG. 26, then an unnatural moving image or blurring results from the perspective of the user. If the scene remains still, the aliasing noise is not as great. In any event, since resolution is always degraded by undersampling, and since the signals of standard television broadcasts and the like are intended for only a limited number of horizontal sweeps per frame, there is a need for a practical way to remove aliasing and recapture the original image with improved quality.